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TPS92692: Conditions for overcurrent protection operation

user4998452
Prodigy 150 points
Part Number: TPS92692

Hello

I have two questions about overcurrent protection operation.

[Q1]
When VIADJ is 0V, does the overcurrent protection become invalid?

[Q2]
If VIADJ drops momentarily while the device is illuminating the LED, overcurrent protection will be effective.
What is the cause of this?
Is there any way to prevent this phenomenon?

Best Regards
Hara

  • 0
    TI__Mastermind 18890 points

    Hi Hara,

    1.  VIADJ will still be valid if VIADJ is 0V.  That being said we recommend that VIADJ be at least 140mV from section 8.3.9 of the data sheet.

    2.  If VIADJ moves fast down and the control loop does not keep up and the LED current is 1.5X the set point from VIADJ then you will get an over current condition.  Is there a way you can filter this VIADJ voltage with a capacitor such that it filters out this transient input.  This can help solve your problem.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 150 points

    Hi Tuan,

    Thank you for reply.

    I have an aditional question about Q1.

    I actually changed VIADJ and confirmed the protection operation when the LED was short-circuited.
    When VIADJ is 0V,the overcurrent protection did not oparate.
    But when VIADJ is over 140mV, the overcurrent protection oprated.

    From this result, it seems that protection does not work when VIAJ is 0V.
    If VIADJ is 0V, do you think  the protection may not work?
    And, If VIADJ is over 140mV, do you think the protection always work?

    Best Regards
    Hara

  • 0 in reply to user4998452
    TI__Mastermind 18890 points

    Hara,

    The protection circuit is enabled at 0V on VIADJ.  If you have VIADJ at 0V then the commanded current is technically 0A  so with shorted LED and 0A you will not see any current therefore you don't get a fault.  At 0V there are offset in the system and at time with certain offset you will see a fault.  Because of this offset and also linearity we recommend VIADJ above 140mV for a reason.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 150 points

    Hi Tuan,

    Actually, when LED short circuit occurred when VIADJ was 0V, a current of 20A or more was flowing.

    From the answer below, I think that overcurrent protection should work, but it didn't actually work.
    Is there any other possible cause besides VIADJ?

    If the cause is that VIADJ is 140 mV or less, it is convincing because it matches the actual phenomenon.

  • 0 in reply to user4998452
    TI__Mastermind 18890 points

    Hello Hara,

    Internally, there is nothing we to disable the Fault circuit when VIADJ is 0V.  We don't recommend VIADJ to be 0V because of offset sensing internally that can cause us not to be able to control or sense the current and that might be what you are seeing.  Again, we don't recommend VIADJ to be below 140mV because of this control and sensing issue.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 150 points

    Hi Tuan,

    I understood.

    Then, we will use it so that it will be 140 mV or more, which is the recommended condition.

    Best Regards
    Hara